1. Field of the Invention
The present invention relates to an optical encoder.
2. Description of the Related Art
In recent years, there has been a progress in size reduction and thinning of encoders. While the size of one side is in a range of 10 mm to 20 mm in prevalent detecting heads, detecting heads of the size of a few mm square are also coming out in the market. In such trend of small sizing, even in packaging, a shape and a form in which small sizing is taken into consideration will be prevailing. For instance, thinning of conventional packages in which a metal or ceramics is used, and replacing the conventional packages by packages in which a packaging technology such as resin moulding is used, is considered to take place.
When the size of a detecting head becomes small, a distance between a light emitting section and a light receiving section inside the head also becomes short. Therefore, a possibility that the proportion of the quantity of light entering the light receiving section via the interior of the head upon being emitted from the light emitting section, occupied in the quantity of detected light becomes high due to a stray light inside the detecting head.
In many cases, an upper portion of an encoder head is covered by glass or sealed by a transparent resin for protecting the detecting head. In such cases, there is a possibility that, light outgoing from a light source enters the light receiving section upon being reflected at an inner surface of the upper portion of the head. Particularly, when the thickness of the encoder head is reduced for making the encoder head small, the angle of reflection at the inner surface of the head generally becomes large, and when the angle of reflection becomes wider than the angle of total reflection, the amount of light entering the light receiving section increases rapidly.
Moreover, regarding light which has undergone multiple reflections at the interior of the encoder head, because of small sizing, more amount of such light tends to enter the light receiving section compared with a conventional head.
This stray light such as the internally reflected light becomes a noise component at the time of generating a position detection signal. The noise component due to the stray light may become a cause of an extreme deterioration of the SN ratio of the position signals. When predetermined signal-amplification is carried out for maintaining an output level such as amplitude of the position signals, there is a possibility that the signal level saturates due to an influence of the large noise component.
When an attempt is made to avoid the signal saturation due to the noise component, there is a need to add an extra process of eliminating the noise component, and there is a possibility that measures have to be taken against the signal deterioration due to the noise component eliminating process.
An example of conventional optical encoders in which, an attention has been paid to the size reduction and thinning is an optical encoder disclosed in Japanese Patent Application Laid-open Publication No. 2005-156549). In FIG. 15, a reflection-type optical encoder which has a light emitting element 402 and photoelectric conversion element arrays 403 and 404 on the side of a head portion 414 is shown. The light emitting element 402 and the photoelectric conversion element arrays 403 and 404 are covered by a transparent optical member 405, and the transparent optical member 405 in a state of covering these components is a packaging member. The light emitting element 402 and the photoelectric conversion element arrays 403 and 404 are disposed on a substrate 401, and the head portion 414 is arranged facing patterns 420 and 421 on a scale disc 430. Conditions such as the following are added in this conventional example. In other words, when a distance from the light emitting element 402 up to the farthest location of the light receiving surfaces of the photoelectric conversion element arrays 403 and 404 is let to be D, a distance from the light receiving surface of the light emitting element 402 and the photoelectric conversion element arrays 403 and 404 up to the surface of the transparent optical member 405 is let to be G, and the angle at which reflectivity at the inside surface of the transparent optical member 405 is 10% is let to be 0, a thickness G is set such that the following expression (A) holds true.G≧D/(2*tan θ)  (A)
For facilitating the size reduction by molding the light emitting element 402 and the photoelectric conversion element arrays 403 and 404 in the head portion 414 by the transparent optical member 405, when a thickness of the transparent optical member 405 which is a protective member, is reduced, light emitted from the light emitting element 402, and reflected light which is reflected at an inner surface of the transparent optical member 405 is incident on the photoelectric conversion element arrays 403 and 404, and the SN ratio of a signal is deteriorated. Therefore, the thickness of the transparent optical member 405 is let to be not less than a fixed value, and the reflectivity from the inside of the transparent optical member 405 is let to be not more than 10%.
In this manner, in the optical encoder disclosed in Japanese Patent Application Laid-open Publication No. 2005-156549, an attempt is made to avoid a problem of internally reflected light by making the transparent optical member 405 substantially thick.
However, in the optical encoder disclosed in Japanese Patent Application Laid-open Publication No. 2005-156549, for making the ratio of a thickness of a resin to a size of the head portion 414 to be not less than a certain constant value, there is a possibility that the disclosed encoder is not satisfactory for thinning of the head. Moreover, by making the transparent optical member 405 substantially thick, it is necessary to have a large working distance of a detection system, and there is a possibility that degrees of freedom in designing will be restricted.
Furthermore, when a mold resin is used for packaging of the detecting head, there is thermal expansion and contraction, and a significant change in hardness of the mold resin due to a temperature change. An example in which, a mold resin is used in the packaging is shown in FIG. 16. In FIG. 16, a head portion 514 is disposed to face a scale 530. In the head portion 514, an LED light source 502, a light receiving element 503, and electrodes 540 and 541 for the LED light source 502, are disposed on a substrate 501, and a glass member 550 is disposed between the light receiving element 503 and the LED light source 502, and the scale 530. The electrode 540 on the LED light source 502 and the electrode 541 on the substrate 501 are electrically connected by a wire 542. The members disposed on the substrate 501 and the members stacked further are covered by a mold resin 505. In an example shown in FIG. 16, when a resin thickness of the mold resin 505 is made large, problems such as breaking of the wire 542, a crack 560 in the mold resin 505, and a split 561 in the glass member 550 tend to occur. Therefore, it becomes difficult to secure reliability, and there is a possibility of an increase in costs for measures to be taken in designing and manufacturing.